There, standing by the door at the big-box store, wal-mart, target, etc., stads the receipt checker.
20 feet from the register, and they ask to see your receipt.
You have a constitutional right to be secure, in your person and papers, from search without probable cause. That means they must have a warrant to search you, as all the stuff you just bought is now YOURS.
Letting them check your receipt or rifle through your belongings is acknowledging that you consent to their unreasonable search.
Think about it.